Fix issue #341 (github).
Add test case for #341 (github).
Variable rename and illustration.
Update document for priority based clearsnippets.
Merge pull request #348 from cwahbong/select-cr-fix

Fix issue #341 (github).
Implements clearsnippets by priority.
Updates the doc for the new clearsnippets behavior. Slight rewording by me.
Refactor: extract methods in SnippetManager.